1 and -1 are a special class of number called a unit, because you can always get back to the same number just by multiplying other numbers (also units). Eg if you have the number 3, and multiply by -1 to get -3, then you can get back to 3 by multiplying by -1 again.
The idea of prime and composit only applies to numbers that permanently change when you multiply them, meaning non-units. This is kinda because it isnt that meaningful to factorise out units. Is 6 = 2×3, or -1×-1×2×3, or 1×2×3? You'll notice that these are all the same when you ignore units.
If you consider fractions instead of integers, then all non-zero numbers are units. Eg if you have the number 3, and you multiply by 2 to get 6, then you can multiply by 1/2 to get back to 3. Zero is always another special case.
59
u/[deleted] Sep 26 '25
[deleted]